The present invention relates to an electrophotosensitive material for use in an image forming apparatus making use of an electrophotographic method, such as an electrostatic copying machine and laser beam printer.
An electrophotographic method such as the Carlson process comprises a step of uniformly charging the surface of an electrophotosensitive material by corona discharge, a light exposure step of exposing the charged surface of the electrophotosensitive material to form an electrostatic latent image on the surface, a developing step of contacting a developing agent with the formed electrostatic latent image to make the electrostatic latent image sensible as a toner image by the toner contained in the developing agent, a transfer step of transferring the toner image onto paper or the like, a fixing step of fixing the transferred toner image, and a cleaning step of cleaning the toner remaining of the electrophotosensitive material after the transfer step.
Recently, in the electrophotosensitive material used in the electrophotographic method as mentioned above, instead of those mainly composed of inorganic photoconductive materials such as selenium and cadmium sulfide which are toxic and are hard to handle, various so-called organic photosensitive materials using less toxic organic photoconductive compounds are proposed. Such organic photosensitive materials are excellent in processability and are easy to manufacture, and are large in the degree of freedom of function design.
Such organic photosensitive materials are often composed of photosensitive layers of function separation type generally comprising a charge generating material for generating an electric charge by irradiation with light, and a charge transferring material for conveying the generated charge.
As the charge generating material used in such electrophotosensitive material, a specific bis-azo pigment is disclosed in the U. S. Pat. Nos. 5,041,349 and 4,999,269. This bis-azo compound is expressed in the following Formula (1): ##STR3## where A.sup.1 and A.sup.2 are same or different, coupler residues, R.sup.1 denotes a hydrogen atom, an alkyl group, an aryl group or a heterocyclic group, and the alkyl group, the aryl group and the heterocyclic group may have a substituent, and n is 0 or 1.
This bis-azo pigment (1) is stable in heat and light, possesses a high charge generation efficiency, and is high in sensitivity and excellent in repeatability.
To prepare, incidentally, an organic photosensitive material of function separation type using charge generating material and charge transferring material, it is necessary to select materials superior in matching, satisfying all electrophotographic properties including the sensitivity, potential retaining performance, potential stability, and residual potential. For example, however, even if the charge generating material may sufficiently generate an electric charge, satisfactory electrophotograhic properties are not obtained unless combined with a charge transferring material capable of injecting and conveying the charge efficiently.
According to the preceding U.S. patents, by combining the bis-azo pigment expressed in Formula (1) with various charge transferring materials (carrier moving substances), it is disclosed that photosensitive materials stable in heat and light are obtained. However, the charge generating materials disclosed in the U.S. patents are, as compared with the ordinary charge generating materials such as phthalocyanine or perylene pigment, fluorene type bis-azo pigment (Japanese Unexamined Patent Publication 57-96345), or oxadiazole type azo pigment possessing a coupler having perinone skeleton (Japanese Unexamined Patent Publication 59-229562), easier to oxidize and deteriorate in ozone, nitrogen oxide NOx and light in the copying machine, and the photosensitive material characteristics are easily lowered. The oxidation and deterioration of such bis-azo pigment (1) may be estimated to be due to decomposition of the azo group by adsorption of ozone on the azo group.
Such oxidation and deterioration will be promoted when the bis-azo compound (1) is used in combination with the charge transferring material which is an electron donor compound. It is considered because the electron donor compound is oriented on the azo group when the basicity of the electron donor compound is strong, and the electron density in the azo group is intensified so as to be vulnerable to the attacks of ozone or nitrogen oxides.
Therefore, it was hitherto impossible to obtain a photosensitive material possessing a high sensitivity and an excellent repeatability without sacrificing the superior characteristics of the bis-azo pigment (1).
Yet, although matching of charge generating material and charge transferring material is satisfactory, if there is a problem in the properties of the binding resin for composing the photosensitive layer by coupling these materials, a photosensitive material comprehensively excellent in electrophotographic properties cannot be obtained. For example, if the strength of the photosensitive layer is not enough or if the adhesion of the photosensitive layer to the base is not sufficient, the surface may be flawed or the photosensitive material may be peeled off due to physical impact receiving received from the cleaning blade pressed to the photosensitive material surface in the image forming apparatus, a felt preventive the toner splash, a charging roller, a transfer roller and other members, or paper contacting with the surface of the photosensitive material at the time of image formation. Therefore, however excellent the sensitivity may be, a spotless excellent image is not obtained, or however excellent the repeatability may be, sufficient durability is not obtained.
As the binding resin, various high polymers disclosed in the foregoing U.S. patents, such as polystyrene, (meth)acrylic ester, polycarbonate, polyester, butyral resin, and epoxy resin, are generally used.
In the Japanese Unexamined Patent Publication 57-4051, the polycarbonate, among the above polymers, is disclosed as the material excellent in film forming capability and capable of forming a tough photosensitive layer superior in resistance to abrasion. However, the polycarbonate is not enough in adhesion with the conductive substrate or base layer, and hence a certain pretreatment is needed prior to layer forming in order to improve the adhesion, which leads to problems in productivity and cost. In the Japanese Unexamined Patent Publications 61-132954 and 2-236555, derivatives of polycarbonate having silicon introduced in the main chain are used as the binding resin, but these derivatives, same as the ordinary polycarbonate, are not sufficient in the adhesion.
In order to eliminate the defects of the polycarbonate and improve the adhesion of the photosensitive layer, the Japanese Unexamined Patent Publication 59-71057 discloses blending of polycarbonate, and the Japanese Unexamined Patent Publication 62-212660 discloses blending of polyester or polyallylate.
In these polymers, however, the main chain is stiff, and the ester bond responsible for adhesion does not act sufficiently on the base such as the conductive substrate. Hence, it is necessary to add a large content to enhance the adhesion, which may lead to lowering of sensitivity of the photosensitive material as the polar group (the electron aspirating group) in the molecule works as a carrier trap, or promotion of photo-oxidation deterioration of the charge generating material and charge transferring material in the high electric field.
In particular, the bis-azo pigment (1) is a molecule not having planeness like the conventional phthalocyanine or perylene pigment, and is high in dissolution in solvent, and the rate of dispersion of one molecule each in the photosensitive layer is relatively high, and hence it is more vulnerable to photo-oxidation deterioration as compared with conventional pigments dispersed in the photosensitive layer as fine particles composed of multiple molecules. Accordingly, the polyestercarbonate or the like cannot be blended in a large quantity, and the adhesion of the photosensitive layer cannot be enhanced sufficiently.
It was therefore impossible to obtain a photosensitive material possessing high sensitivity and repeatability without sacrificing the excellent characteristics of the bis-azo pigment (1).